The present invention is concerned with multi-component picture or video coding such as the coding of color pictures or videos.
Colored images and videos are represented using a so-called color space with either three or four dimensions and the dimensions are also referred to as components. An example for a three-component color space is the R′G′B′ color space. In this color space, the primaries red (R′), green (G′), and blue (B′) form the basis of the three dimensional color cube. In the signal processing application space, however, it is crucial to minimize the correlation between the different components in order to achieve high efficiency in terms of power consumption, implementation costs, or compression efficiency in the case of image and video compression. Therefore, R′G′B′ signals are often converted to Y′CbCr with the first component (Y′) being referred to as the luma and the remaining two components (CbCr) being referred to as the chroma components. In contrast to the R′G′B′ color space, the chroma component values of Y′CbCr represent a color difference relative to the blue or the red primary. Consequently, the chroma energy is often reduced and hence leads to higher compression efficiency. However, due to the application of a fixed transform, i.e., the R′G′B′ to Y′CbCr conversion, the resulting output is often not locally optimal. A possibility to overcome such kind of limitations is the application of a local prediction between the color components. Such a prediction is often referred to as inter-component prediction (ICP). The ICP can be applied to both R′G′B′ and Y′CbCr signals. In the first case, the ICP would result in an energy reduction for the chroma components, and hence, it can be treated as a replacement for the external color space conversion. In the second case, the ICP approach can be treated as a further decorrelation step between the different color components, and hence, the ICP approach results in higher compression efficiency. For the sake of simplification, regardless of the native or input color space, the remaining description refers to luma or Y′ to denote the first and the main component and first or second chroma, or respectively Cb or Cr, to denote the two remaining chroma components. It is important to note that the order of the chroma components can be important as the processing order can be sequential for some applications.
ICP is, if ever, applied pair-wise, such as for example by predicting one chroma component on the basis of the luma component and doing the same for the other chroma component. It would be favorable, however, to be able to further increase the coding efficiency.